U.S. Pat. No. 3,566,637 of Hallman discloses various arrangements of magnetic lock and key. Each such lock has a number of permanently magnetized elements which must be moved by a magnetic field associated with the key into predetermined positions before the lock can be operated to drive an associated device. The first form of key mentioned in the Hallman patent consists of a flat, elongated piece of magnetic material which is magnetized to present at each major face of the piece of material a number of magnetic poles. For each such pole, there is a corresponding pole of unlike polarity occupying a corresponding position on an opposite major face of the piece of magnetic material. Thus, the piece of magnetic material behaves as an assembly of bar magnets with the respective poles of each bar magnet spaced apart in the same direction, that is the direction extending between the major faces of the piece of magnetic material. The Hallman patent also mentions that the key may be a strip of support material to which permanent magnets are attached.
A third form of key disclosed in the Hallman patent aforesaid is circular, that is the key has the form of a cylindrical pin. This key is stated to be formed with transversely extending magnets. FIG. 4a of the drawings of the Hallman patent shows an example in which two of these magnets present like poles which are adjacent to each other. This configuration can be achieved by insertion of pre-formed bar magnets into respective sockets in a key body but could not be achieved by magnetizing a single piece of magnetic material.
In each of the keys disclosed in the Hallman patent, it is inevitable that, for each magnetic pole on the key, there will be a corresponding pole of unlike magnetic polarity occupying a diametrically opposite position. This restricts the number of different patterns of magnetic poles which could be provided in mechanically similar keys and restricts the scope of application of the keys.